I'm not sure whether to get the 4890 BE or the 275 OCX (in case some of you are not aware of the OCX, it is clocked about 35Mhz HIGHER than EVGA's SSC version (709Mhz vs 674Mhz), and is only £9 more than the HD 4890 black)..
I almost defo will be adding another one. If I get the GTX 275 OCX's, I will have to get Core i7 as there are currently no AM3 SLI boards, and I don't want to use an AM2+ motherboard because backwards compatabilty reduces performance. If I get the 4890's I will get AMD Phenom II X4 955. However I would rather get the AMD setup because its cheaper.. But it soley depends on the graphics cards.
I couldn't find any review on the GTX 275 OCX, but some review say that the 4890 BE ban outperform the GTX 285 in some tests.
I'd look very, very carefully into the motherboard/case first.
Either of those combinations is going to produce a lot of heat and most motherboards do not seperate the cards enough to allow the upper card to get enough cooling air and the last thing an overclocked card needs is to have its cooling system choked.
As for cards: I like the GTX275.